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Are all amps being built wrong?

The power amplifiers that drive our loudspeakers are mostly built as a low impedance voltage source. They have always been ... but why?

Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)

So ... why are amplifiers not built as a high impedance current source?

This is an interesting read: https://www.current-drive.info/
rudyb

Showing 10 responses by jasonbourne52

jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Anybody remember what issue that was?
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Nelson Pass designed and sold through First Watt a current-drive amp. I remember reading a review of it in Stereophile years ago. Apparently it's major flaw was an output impedance of some 50 ohms! 
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
There is a purity and transparency of the sound of the Futterman's that must be heard to be believed! I first heard a friend's pair driving Quad 57's back in 1978 and was instantly convinced that the Futterman OTL's are world-beater amps! 
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Because a current drive amp is only useful with a speaker that has a flat impedance. Otherwise the amp will act as a tone control and follow the dips and peaks of the speaker's impedance curve. This can hardly be called "accurate"!
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
I have three Futterman OTL's. Two mono's and a stereo chassis version that Julius made and sold as the Harvard Music H3 in the mid-Sixties. That one got a review in the old High Fidelity mag! The H3 I bought has an added pair of Altec autoformers because the first owner used it to drive a large custom-made pair of panel speakers that had ten oval cone drivers per side! Consequently these presented a low impedance that the stock H3 couldn't drive - hence the Altec autoformers!
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Look up Ohm's Law!
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Julius Futterman solved the tube amp feedback problem by eliminating the output transformer. His OTL amps can have 60 db feedback with low output impedance and low harmonic/IM distortion. Yet remain 100% stabile into complex loads! The drawback is that they are best used with speakers that are 8 ohms and up due to the fact that power/watts declines into low impedances - but rises with impedance increase!
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
The solution is to add an autoformer to the Futterman OTL's output. Say, 16 to 32 ohms. This will allow low impedance drive. And a corresponding increase in power/watts from 60 to 120 to 240 from a sextet of 6LF6 output tubes! 
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Miyajima in Japan copied the Futterman circuit and made some mono amps. The late Art Dudley tested and reviewed a pair for Stereophile some years back.
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Julius used hollow ferrite rods on the wiring of his amps - at least the one's I've seen. 
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